1. Field of the Invention
The present invention relates to a motor drive apparatus, and in particular to a motor drive apparatus having the function of suppressing the temporal variation of regenerative current.
2. Description of the Related Art
During deceleration of a motor, the motor acts to provide regenerative braking. Power regeneration is a known method of handling power when regenerative power is large (for example, refer to Japanese Unexamined Patent Publication No. H06-062584). A converter used for power regeneration is shown in FIG. 1. The converter 1001 used for power regeneration comprises power devices Tr1 to Tr6 as transistors each of which is connected in parallel with one of diodes D1 to D6. Power generated by a motor is fed to charge a smoothing capacitor (not shown) provided between terminals 1003 and 1004, and the power stored on the smoothing capacitor is inversely converted by the converter 1001 and returned to a power supply 1002. Known methods for controlling the power regeneration converter 1001 include a PWM control method and a 120-degree conduction method. In the PWM control method, regenerative current can be made to have a sinusoidal waveform, and the amount of its temporal variation can be controlled. However, the switching noise increases with the PWM operation, and the cost increases as a result of adding a filter or the like. Accordingly, the 120-degree conduction method is generally preferred for use.
The 120-degree conduction method is a method that detects the phase of the supply voltage and that returns the regenerative power to the power supply only during the 120-degree period in which the potential difference of the supply voltage is large. FIGS. 2A and 2B show a temporal variation of power phase, timing charts illustrating the ON/OFF operations of the power devices Tr1 to Tr6, and temporal variations of R-phase, S-phase, and T-phase currents when the 120-degree conduction method is used. FIG. 2A shows the case when the difference between DC link voltage and supply voltage is large and FIG. 2B shows the case when the difference between DC link voltage and supply voltage is small.
As shown in FIG. 2A, when the difference between DC link voltage and supply voltage is large, periods (indicated by hatching) are provided during each of which switching is performed at a given frequency to control the current so as not to exceed a given peak current. In this case, di/dt (the amount of temporal variation of current) is calculated in accordance with the following equation (1), and inrush current is calculated by multiplying di/dt with time.
            d      ⁢                          ⁢      i              d      ⁢                          ⁢      t        =            (                        V          DC                -                              V            IN                                3                              )        L  
where VDC is the DC link voltage, VIN is the supply voltage, and L is the inductance of AC reactor and power supply.
In the conventional art, when the difference between the DC link voltage and the supply voltage (DC link voltage-supply voltage) is large, inrush current occurs during power regeneration, as in the R-phase current at time t1 shown in FIG. 2A. Accordingly, if the power devices are to be protected in the conventional art regenerative method, di/dt must be suppressed by increasing the inductance L of the AC reactor. In the case of the PWM control method, the occurrence of inrush current can be suppressed without increasing the inductance of the AC reactor, but there in turn is the problem that the temperature of the power devices increases.